1. Field of the Invention
This invention relates to an improvement in the field of photographic silver halide imaging systems and, particularly, to novel silver halide photographic imaging systems employing reduced amounts of photoactive silver halide in conjunction with a chemically bleachable colorant to provide increased image density. These systems are useful in applications in which silver halide photographic elements are used and are particularly useful in X-ray films and graphic arts films, e.g., lithographic films, among others.
2. Description of the Prior Art
Unlike the present invention, photographic silver halide elements of the prior art rely entirely on developed silver to form an image, or in the case of color films, on dye formed imagewise in or near the silver halide layer, the formation of which is catalized by the development of the exposed silver halide. Such elements are not suited to some uses, may require long development times in the case of color films, and may have low transmission density and low or moderate covering power as measured by transmission density. Attempts have been made to produce silver halide photographic films which have high covering power and which therefore require less silver halide to produce an image, e.g. U.S. Pat. No. 3,413,122 and references cited therein. In that patent an element is described having a silver halide emulsion layer and an inner emulsion layer containing unfogged internal silver halide grains. In such an element the inner layer has a very low optical density and no image until an image is formed in it by bringing up the optical density imagewise by development, thereby relying on the nature of the material of the inner layer to be able to develop sufficient image density. Such elements can generate silver images having increased covering power but are still limited to covering power obtainable by development of a silver halide emulsion in situ.
Other elements of the prior art include those having a silver halide layer and an antihalation layer as in U.S. Pat. No. 1,971,430. The antihalation layer was not used as an image-forming layer, and such elements were neither designed for nor used in a process of imagewise bleaching of a colorant layer to produce an image in that layer.